A steel sheet for automobile is required to improve fuel efficiency and crashworthiness. Accordingly, attempts are being made to increase strength of the steel sheet for automobile. However, ductility such as press formability generally decreases in accordance with the improvement of strength, so that it is difficult to manufacture a component having a complicated shape. For example, in accordance with the decrease in ductility, a portion with a high working degree fractures, or springback and wall warp become large to deteriorate accuracy in size. Therefore, it is not easy to manufacture a component by press-forming a high-strength steel sheet, particularly, a steel sheet having tensile strength of 780 MPa or more.
Patent Literatures 1 and 2 describe a forming method called as a hot stamping method having an object to obtain high formability in a high-strength steel sheet. According to the hot stamping method, it is possible to form a high-strength steel sheet with high accuracy, and a steel material obtained through the hot stamping method also has high strength. Further, a microstructure of the steel material obtained through the hot stamping method is substantially made of a martensite single phase, and has excellent local deformability and toughness compared to a steel material obtained by performing cold forming on a high-strength steel sheet with multi-phase structure.
Generally, crushing strength when collision of an automobile occurs greatly depends on material strength. For this reason, in recent years, a demand regarding a steel material having tensile strength of 1.800 GPa or more, for example, has been increasing, and Patent Literature 3 describes a method having an object to obtain a steel material having tensile strength of 2.0 GPa or more.
According to the method described in Patent Literature 3, although it is possible to achieve the desired object, sufficient toughness and weldability cannot be obtained. Even with the use of the other conventional techniques such as steel sheets described in Patent literatures 4 to 6, and the like, it is not possible to obtain tensile strength of 1.800 GPa or more while achieving excellent toughness and weldability.